1. Field of the Invention
This invention relates generally to the removal of total cyanide from aqueous solutions and, more particularly, to the removal of total cyanide from groundwater and industrial wastewater streams.
Primary aluminum metal is produced in electrolytic cells with carbon cathode or potlining. A by-product in the production of primary aluminum metal is water soluble complexed iron-cyanide. These complexed iron-cyanides similar to Fe(CN).sub.6.sup.-4 are formed by the reaction of carbon in the potlining, nitrogen in the atmosphere and iron present as potlining electrical conductor bars and the pot shells. Unlike simple, "free" cyanides, the complexed cyanides do not dissociate readily and are therefore non-toxic. Also unlike simple, "free" cyanides, complex cyanides are very difficult to treat and remove.
Past practice common to the aluminum industry of storing spent potlining in an outdoor environment has resulted in the leaching of these complexed cyanides into the groundwater. This then becomes an environmental issue and must be dealt with according to state law and the Federal regulations of the Environmental Protection Agency (EPA).
2. Description of the Prior Art
Lime precipitation/coagulation is one of the oldest forms of chemical treatments used to treat wastewater. The lime precipitation/coagulation method dates back to the late 1800's. In this process, the calcium ion from the lime reacts with soluble anions to form insoluble compounds (e.g., calcium phosphate and calcium fluoride) which settle out of solution and can then be physically removed. Additionally, alkalinity imparted to the water as hydroxyl ions tends to coagulate certain organic compounds and colloidal materials. The net result is the removal of a wide range of materials from the treated water in the form of sludge.
This invention treats total cyanide by chemical precipitation/coagulation utilizing lime and ferrous iron salt.
Soluble [Fe(CN).sub.6 ].sup.-4 ion reacts with ferrous iron to form a precipitate as follows: EQU [Fe(CN).sub.6 ].sup.-4 +FeSO.sub.4 .fwdarw.Fe[Fe(CN).sub.6 ].sup.-2 +SO.sub.4.sup.-2 ( 1) EQU [Fe(CN).sub.6 ].sup.-4 +2FeSO.sub.4 .fwdarw.Fe.sub.2 [Fe(CN).sub.6 ]+2SO.sub.4.sup.-2 ( 2)
The formation of the Fe.sub.2 [Fe(CN).sub.6 ] precipitate (Berlin White) is dependent upon the amount of available ferrous iron and the pH of the water.
The [Fe(CN).sub.6 ].sup.31 4 ion can also react with ferric iron to form Prussian Blue precipitate as follows: EQU 3[Fe(CN).sub.4 ].sup.-4 +4Fe.sup.+3 .fwdarw.Fe.sub.4 [Fe(CN).sub.6 ].sub.3( 3)
This reaction occurs under slightly acidic conditions.
Various processes have been developed specifically for the removal of both total and free cyanide from wastewater. The term "total cyanide" refers to all of the different forms of cyanide that exist in an aqueous solution. However, most of these processes utilize multiple reaction vessels and require adjusting the pH of the wastewater to very basic or acidic levels, i.e., high or low pH, during treatment. The initial capital cost of constructing a multiple reaction vessel facility increases with each reaction vessel required. Further, the pH levels generally required for treatment of the wastewater require special and expensive precautions to be taken to prevent corrosion damage to the reaction vessels and associated piping and also pose a safety hazard for workers who must operate and maintain the treatment facility.
One example of a known treatment process for the removal of cyanide from wastewater consists of two individual reaction stages, a lime treatment stage and a ferrous/ferric iron treatment stage. In this known process, lime is mixed with the wastewater in a first reaction vessel to increase the wastewater alkalinity. After settling, the supernatant from the first treatment vessel is introduced into a second treatment vessel in which the pH is adjusted with sulfuric acid to a range of between 3 to 5. The ferrous/ferric iron reagents are added to the second reaction vessel and the mixture is agitated. A polyelectrolyte solution is then added to the mixture and settling occurs in the second reaction vessel. The supernatant is withdrawn from the second reaction vessel and is filtered as a treated effluent before disposal. Available data reporting the cyanide concentration in treated groundwater using this known process shows a wide variability of treatment performance. Variations in cyanide concentrations of 1.1 to 13.0 mg/l in the treated effluent are common. This variability in cyanide removal levels is believed to be an inherent shortcoming in this prior two-stage process resulting specifically from the colloidal characteristics of the reaction mixture and the small quantity of iron-cyanide precipitate formed therein. In addition, the potential corrosion problems in operating a treatment facility at pH levels between 3 to 5 make this prior process relatively expensive and environmentally unfriendly, making it unattractive for use in a full-scale plant environment. U.S. Pat. No. 4,543,189 to Rice et al. discloses a method for removing complexed zinc-cyanide from steel mill wastewater such as is found in blast furnace blowdown water. Ferrous ions are added to the wastewater in a first reactor vessel along with acid to control the pH of the water. The wastewater is then transferred to a second reactor vessel in which the pH is further adjusted to cause cyanide to precipitate as Prussian Blue. Additional water treatment processes are disclosed in U.S. Pat. Nos. 3,147,213; 3,847,807; 4,176,060; 4,312,760; 5,015,396; 5,055,199; 5,093,007; 5,106,508; 5,160,637; and 5,290,455.
It is an object of the invention to provide a total cyanide treatment apparatus and method which solves the problems present in the prior art. The present invention provides an apparatus and a method for reducing cyanide to very low levels on a consistent basis while employing only a single reactor vessel. In addition, the process of the present invention operates at nearly a neutral pH which minimizes corrosion problems and eliminates the need for expensive construction materials.